This invention relates to dynamoelectric machine excitation systems such as brushless exciters for synchronous motors.
FIG. 1 shows a synchronizing circuit for a synchronous AC motor for supplying DC excitation from a full wave bridge rotating rectifier 10 to the rotor field winding 12 of the motor. A pulse or synchronizing module B is configured for the control of SCR2.
The present invention has particularly to do with improvements in the gate module A for the control of the turning on of SCR1.
During the synchronizing process of an AC synchronous motor, the rotor sees a very large flux wave produced by stator winding 14. Since the effective area of the rotor is large, the number of turns of the stator winding is large, and the permeability of the rotor is high, this field is capable of inducing several tens of thousands of volts in the winding 12 of the rotor. Such a voltage would be capable of destroying the rotor insulation if steps were not taken to avoid it.
In practice, a resistive load, such as resistor R of FIG. 1, is placed across the rotor winding 12 to reduce this large voltage by allowing current to flow. Resistor R cannot be left in the circuit following synchronization, however, since it would dissipate considerable power and not endure for a long life.
In order effectively to remove the resistor R from the circuit following synchronization, it is connected to the rotor winding by means of a diode D and a thyristor type device SCR1 connected in inverse parallel relationship. The diode D allows current to flow easily in one direction while blocking positive voltages which will normally be in the range of 30 to 150 volts DC while the motor is running in synchronism. SCR1 is intended to be triggered to conduct when the positive voltage is greater than approximately 200 volts. This voltage is greater than the maximum DC supply voltage available but it is also less by a considerable margin than the smallest voltage likely to cause damage to any of the insulation of winding 12.
The above-mentioned voltages are by way of example. More generally, the present invention has as an objective to provide an improved gate module for an excitation system in which the controlled voltage across the SCR is at least about 100 volts.
The gate module A is a device or circuit which senses the voltage on the anode of SCR1 (at circuit point TP) and delivers a gate pulse to gate electrode G to start conduction upon that voltage reaching a predetermined trigger level. Previous gate modules have been very simple circuits such as that illustrated in block A of FIG. 2. It utilizes a Zener diode Z to provide the voltage sensing function and, after breakdown of the Zener diode, to supply current to the gate electrode G of SCR1 to trigger the SCR. A resistor RG is connected between the gate electrode G and the cathode electrode C. The Zener diode Z produces very poor gate drive since the voltage used for triggering collapses as soon as the SCR starts to conduct. Also, this type of gate module cannot be expected to deliver the steep rising pulse necessary to cause the SCR to quickly go into conduction over its entire junction area. Soft triggering of an SCR is known to be likely to produce premature failure of the device. Therefore, what is needed is an improved gate module that provides a vigorous gate pulse capable of causing SCR1 to go quickly into conduction over its entire area and to do so with a circuit that is capable of the dense packaging required for use in brushless excitation systems with a high degree of economy and reliability.
Reference is made to Linear/Switchmode Voltage Regulator Handbook, a 1982 publication of Motorola, Inc., by J. Alberkrack et al., pages 121 through 134, for description of SCR crowbar overvoltage protection circuits and particularly the use of modern electronic integrated circuits such as that designated type MC3423 in such circuits.
The present invention utilizes an integrated overvoltage protection circuit for precise sensing of anode voltage of SCR1 and to provide a vigorous gate pulse when provided with energy storage in a capacitor to prevent the sudden collapse of the voltage available for triggering the SCR, resulting in more uniform triggering pulses. In contrast to prior known applications of such integrated circuits, the present invention is adapted to the particular requirements of synchronous motor excitation systems in which the controlled voltage is at least about 100 volts and typically approximately 200 volts, in a gate module that provides the compact packaging, high economy and reliability required of brushless motor excitation systems.